When weaving a fabric on a weaving machine, the warp threads are positioned with respect to the level at which a pick thread is introduced in each weaving cycle during the successive weaving cycles. This positioning of warp threads is referred to as the shed formation. The positions of the warp threads in the successive weaving cycles are in this case determined in such a manner that the weaving process results in a fabric having a predetermined weaving pattern. This positioning of warp threads with respect to the pick introduction level on a weaving machine is automatically realized by means of a shed-forming device.
With a known jacquard shed-forming device each warp thread to be positioned passes through a heddle eyelet of a heddle. At the bottom, each heddle is connected to a retracting spring which exerts a downwardly directed force on the heddle and, at the top, is connected, via a harness cord, to the end of a tackle cord of a tackle system, which end is situated at a higher level. The tackle system comprises two hooks which are displaceable in the vertical direction. The position of the hooks determines the height of the end of the tackle cord.
Each hook can be displaced in the vertical direction by a respective knife. These two knives are driven so as to move in a reciprocating manner in opposite phases with respect to one another. Each hook comprises an elastically deformable strip which can be placed in a selection position by means of an electromagnetic selector in which the hook is not caught by its knife and kept at a fixed height (is selected). If a hook is placed in the non-selection position, it is caught by a reciprocating knife.
By selecting or not selecting the respective cooperating hooks, a warp thread can, via the tackle cord and the heddle, be brought to the required position during the successive weaving cycles in order to produce the desired fabric.
There are also shed-forming devices which comprise a holding element with a flexible strip or a rotating pawl which can be positioned with respect to a hook by an electromagnetic selector. This strip or pawl of the holding element can then be brought into a selection position in which the holding element keeps the hook at a fixed height, or can be brought into a non-selection position in which the hook is caught by a knife. In this case, the holding element thus acts as a selection element.
With a number of known shed-forming devices, the selection of the hooks is carried out by means of a selection device which has the features described in the first paragraph of this description.
Jacquard machines comprise large numbers of these shed-forming devices. There is an increasing market demand to provide jacquard machines with ever larger numbers of shed formation elements, while at the same time keeping the so-called footprint (the perpendicular projection on a horizontal plane) of the device limited. The reduction in the footprint of the hook-selecting devices offers a solution. A reduction in the dimensions of the knives in the longitudinal direction is in this case the most advantageous, as this makes it possible to reduce the number of knives or the height of the knives since the span between the suspension points can be reduced. This reduces the inertia of the components of the shed-forming device which are to be driven, as a result of which the shed-forming device can be produced more inexpensively.
With the shed-forming device according to EP 0,119,787, the selection device comprises a solenoid, the longitudinal axis of the coil of which extends in the longitudinal direction of the knives. At the position of the end faces of the core, thin plates are provided which are bent towards one another. The plates act as magnetic poles by means of which a reciprocating hook of a shed-forming device can be placed in the selection position. This selection device has a relatively large footprint. Reducing the coil length is not the solution to this problem, as this limits the space for the windings. The smaller the space for the copper, the higher the energy consumption in order to achieve a desired attractive force. Compensating by increasing the coil diameter is not a solution as this increases the footprint in the other horizontal direction. In addition, the resistance increases significantly with a larger coil diameter, thus requiring more power. The poles in this case also extend along a relatively large length which results in a relatively large magnetic leakage loss. As a result of miniaturisation, these plates come to lie closer together so that the magnetic leakage flux increases significantly. This is one of the reasons why the efficiency is low, in the sense that a relatively large amount of power is required in order to achieve a certain attractive force.
EP 0,214,075 describes a selection device with a vertically arranged coil which is designed to magnetically influence a thin strip of elastic material so that it is brought into a selection position by elastic deformation. This device also requires a relatively large amount of electrical power in order to achieve an efficient attractive force. In addition, this selection device has a limited speed of response.
Materials which have excellent elastic deformability and resistance to fatigue, such as spring steel, offer poor magnetic conductivity. In order to reduce the amount of electrical power required, a material having a better magnetic conductivity could be chosen, but these materials then have the drawback that they have a smaller resistance to fatigue, causing them to break more quickly. This results in a selection device which does not meet today's requirements in terms of reliability and service life.
With many selection devices, it is also the case that the positionable part cannot be made very thick as this would cause the inertia and the restoring force required to be too high. The limited thickness and the choice of material together restrict the number of magnetic field lines which can run through the magnetically influenceable zone of the selection element.
Due to the demand for increased production speeds, the selection devices have to have higher speeds of response and greater reliability has to be achieved.